1. Field of the Invention
The present invention is directed to a method and an apparatus for continuously monitoring the concentration of a metabolite, such as glucose or lactic acid, in biological tissue, in which a perfusion fluid is fed to a microdialysis probe, preferably implantable in subcutaneous tissue, and removed therefrom as dialysate after enrichment with the metabolites contained in the lymph, in which an enzyme is added to the dialysate and/or perfusion fluid, and in which the concentration of the metabolite in the dialysate is determined under the selective effect of the enzyme at a measuring point, which is positioned ex vivo, using an electrochemical sensor.
2. Discussion of the Prior Art
Methods and apparatus of this type are primarily used in the field of human medicine. Apart from applications in sports medicine such as monitoring lactic acid during anaerobic strain or the determination of the variation of free fatty acids under different conditions, it is foremost the monitoring of glucose of diabetics which is of interest. Rapid information about the level of glucose in the blood is an essential condition for an effective and controlled administration of insulin. Under non-clinical conditions of everyday life, a self-monitoring of the glucose level by the patient using biosensors continually connected to the venous bloodstream is hardly possible, due, among other things, to problems of coagulation. The determination of the glucose concentration in the interstitial lymph, which is mostly linearly correlated to the glucose level in the blood, has proven to be a good alternative. In the earliest methods working with this principle (cf. EP-A 275 390), a needle sensor coated with an immobilized enzyme was introduced into the tissue and the glucose level was determined without taking samples in situ directly in the interstital fluid by measuring on an enzymatic-electrochemical basis. This concept proved to be disadvantageous, though, since the glucose sensitivity of the needle sensor continually decreased due to influences of physiological parameters during long-term measurements. In order to avoid this, it was proposed (DE-A-41 30 742) to extract metabolites from the lymph by means of a microdialysis technique, and to perform the actual measurement ex vivo, wherein transport of the samples from the subcutaneously implanted microdialysis probe to an electrochemical enzyme cell positioned outside the body is performed by way of a dialysate tube. When additionally rinsing the enzyme cell with a buffer fluid, this technique made it possible to diminish the influence of enzyme inhibitors present in the biological material and to improve the validity of long-term measurements. Intravenous reference measurements showed a high correlation and, under normal conditions, a small time delay between the glucose levels of blood and tissue. A drift of the measuring signal connected to the gradual loss of enzyme activity was observed, though. At the same time, the manufacturing and exchange of the enzyme-covered sensor caused comparatively high material expenditures.
In order to avoid these disadvantages, it was proposed in a method as described above (WO 89/027290) to add an enzyme to the dialysate and/or perfusion fluid flow, in order to guarantee a constant enzyme activity. In this, the enzyme is added to the perfusion fluid flow either as a slowly dissolving tablet or by diffusion through a hollow fibre. In this method of enzyme addition there is the danger, though, that the enzyme reaches the tissue of the patient, where it can lead to strong inflammation of the tissue. Furthermore, when there is a predetermined solubility profile of a tablet or a fixed diffusion rate through a hollow fibre, there is no possibility to adapt the concentration of the enzyme to different dialysate flows or to vary the concentration of the enzyme in a fixed dialysate flow.